Handheld computing devices typically use standard battery chemistries including ni-cad, lithium-ion, and nickel-metal hydride. In order to recharge these batteries, operators may use standard recharging options such as, for example, conventional AC (alternating current) outlets. However, mobile users who are in remote locations oftentimes do not have access to conventional AC outlets. As a result, they oftentimes have no way of recharging the batteries of their handheld computing devices.
Recently, solar power has been used to power up a handheld device. As demands for the power of the handheld computing devices increase, it becomes more important to provide stable power to the devices. However, given the characteristics of the solar cells that provide solar power, it is relatively difficult to track the solar power drawn from the solar cells to maintain relatively stable solar power output.
FIG. 1A is a diagram illustrating a model circuit of a typical solar cell. As shown in FIG. 1A, the I-V equation for the diode part of the model can be written as follows:
      I    D    =            I      o        ⁢          ⅇ                        qV          CELL                mkT            The I-V curve for the cell may be described as follows:
                              I          CELL                =                              I            Q                    -                                    I              o                        ⁢                          ⅇ                                                qV                  CELL                                kT                                                                        [        1        ]            Similarly, the V-I curve may be described as follows:
      V    CELL    =            kT      q        ⁢          (                        ln          ⁡                      (                                          I                Q                            -                              I                CELL                                      )                          -                  ln          ⁡                      (                          I              0                        )                              )      
For a typical cell the Cell current is about 1 Amp at 650 mV so I0 can be computed to be 1.389×10−11. FIG. 1B is a diagram illustrating an example of the V-I characteristic of a solar cell. The output is similar to a current limited voltage source. The power out of the cell at any given point on the V-I curve is the voltage times the current. FIG. 1B also includes a plot of the available cell power plotted as a function of voltage. As shown in FIG. 1B, there is a fairly sharp peak operating power that is the desired operating point for maximum power out.
FIG. 2A is a schematic diagram illustrating a typical solar power system using a boost switching regulator and a storage battery. Referring to FIG. 2A, the boost regulator would be used in low cell count systems where the battery voltage is larger than the available cell voltage. The boost regulator boosts the solar cell voltage to a voltage suitable for a conventional battery charger. A controller monitors the current into the battery charger and controls the current drawn by the charger to control the power draw from the solar cell. Since the output voltage is constant the power to the battery charger is proportional to the current drawn so the control be considered to be a power control and the solar cell sees the converter as a adjustable constant power load as illustrated in FIG. 2B.
FIG. 3A is a diagram illustrating a cell V-I source plot with a resistor load line and some constant power load lines. Referring to FIG. 3A, the resistor load is always stable since both the source and load resistances are positive. The constant power loads are conditionally stable. The 600 mW load is always unstable because there is no intercept with the cell V-I curve. The 400 mW and 500 mW loads are stable at the Intercept B locations because the positive conductance of the cell is greater than the negative conductance of the load. These loads are unstable at Intercept A. With the 600 mW load the load will continue to demand current that the cell cannot supply so the cell will go into constant current mode and the cell voltage will go down. This similar situation will apply to the other two loads if cell voltage is below Intercept A; however, if the cell voltage is above Intercept A, the cell voltage will increase and finally settle at Intercept B.
FIG. 3B is a diagram illustrating a SPICE simulation result that shows the behavior of the system when the load current is stepped up in 0.5 mA steps from a load current of about 75 mA. At each step the cell voltage drops by an increasing amount and when the peak power point is exceeded so there is no intercept, the system collapses. The controller in FIG. 2A must sense the impeding collapse and recover before the actual collapse occurs.
In addition, a conventional portable device or handheld device typically includes a battery and an AC adaptor for charging the battery. Certain handheld devices, such as a calculator, include a solar panel to generate solar power to activate the device. However, such a device does not normally include other power sources to charge the battery. Sometimes the solar power source or AC outlet may not be conveniently available. In such circumstances, a device limited to one charging method may not function properly.